1. Field of the Invention
The present invention relates to the tuning of a phase-locked loop (PLL,) and the demodulation of a modulated sound carrier and is particularly applicable to frequency modulated (FM) sound demodulation within a television receiver system. Such a system has a plurality of carrier frequencies that typically extend over the range of 4.5 MHz to 6.5 MHz:
2. Discussion of the Related Art
FIG. 1 represents a block diagram of a conventional FM demodulation PLL. This PLL includes a mixer 100, sometimes referred to as a multiplier, a low pass filter 110, a buffer 120, an amplifier 130 and a voltage controlled oscillator (VCO) 140. The mixer 100 receives and mixes the signals f.sub.IN and f.sub.VCO that are respectively present on its inputs 150 and 160; input 160 being the output of the VCO 140. The output 170 of the mixer that carries the demodulated audio signal f.sub.D is passed through the filter 110 and then the buffer 120 and the output 180 of the buffer is connected to the inputs of the amplifier 130 and the VCO 140.
The FM input signal f.sub.IN is a composite signal that can correspond to any one of a plurality of carrier frequencies that are spread over a large frequency range. The problem associated with the system illustrated in FIG. 1 is that, because the carrier frequencies are spread over a large frequency range, the VCO 140 has to have a high gain, typically several MHz/Volt, so as to allow the PLL to lock onto any one of the plurality of carrier frequencies. Thus, carrier frequencies that are spread over a large dynamic range require a VCO that has a high gain and a frequency range that is greater than the carrier frequency range. However, the use of a high gain VCO results in a demodulated audio signal f.sub.D that has a small amplitude; typically in the range of tens of millivolts, which results in a demodulated audio signal f.sub.D that has a poor signal to noise ratio (SNR). The demodulated audio signal f.sub.D must then be amplified by the amplifier 130 to obtain a larger `more workable` signal. However, the undesired noise will also be amplified by the same amount as the desired audio signal. Therefore, the respective SNRs at the input and the output of the amplifier 130 will be substantially the same. Furthermore, a high gain VCO will suffer from poor linearity characteristics that will ultimately affect the quality of the demodulated audio signal f.sub.D.
Ideally, the required audio signal, that is superimposed on the carrier signal, should be demodulated by a VCO that has a low gain, so that the demodulated audio signal f.sub.D has an amplitude of some hundreds of millivolts; this would then ensure a good SNR.
Therefore with the system arrangement of FIG. 1, a conflict arises between the VCO gain requirements for its tuning and demodulation functional modes. It should be noted that the gain versus frequency response of the system of FIG. 1 is substantially static.
A object of the present invention is to provide a demodulation PLL that has an improved signal-to-noise ratio.
Another object of the invention is to provide a demodulation PLL that has improved demodulation linearity characteristics.